1. Field of the Invention
The present invention relates to eyeglass frame and, in particular, such frames using Ti-Ni alloy elements.
2. Description of the Prior Art
U.S. Pat. No. 4,772,112 issued to Zider et al. and assigned to CVI/Beta Ventures, Inc. discloses to use a shape memory alloy such as Ti-Ni alloy for elements in the eyeglass frame.
It is well known in the art that the Ti-Ni shape memory alloy has a super elasticity or a pseudo elasticity as well as the shape memory effect. The reference 1 proposes to use, as an optimized elasticity, a combination of a super elasticity as shown in FIG. 2f and a work-hardened pseudo elasticity of FIG. 2h in the Reference.
The stress-strain curve according to the optimized elasticity has a comparatively small hysteresis loop and the strain variation is almost in propotion to variation of the stress. Although the reference recommends utilization of the optimized elasticity for the eyeglass frame, it has been confirmed by the present inventor that the super or pseudo elasticity having a generally rectangular hysteresis is more relevant for the eyeglass frame elements such as bridges and temples.
Although the conventional Ti-Ni shape memory alloy has the stress-strain curve of the rectangular hysteresis loop as shown in FIG. 2c in the reference, the curve is realized only a limited temperature range which is smaller than a broad relevant temperature range such as 0.degree.-20.degree. C. Therefore, the Reference abandon utilization of the pseudo elasticity having the generally rectangular hysteresis for the eyeglass frame elements.